Entity functions

In this article

Entity functions define operations for reading and updating small pieces of state, known as durable entities. Like orchestrator functions, entity functions are functions with a special trigger type, the entity trigger. Unlike orchestrator functions, entity functions manage the state of an entity explicitly, rather than implicitly representing state via control flow.
Entities provide a means for scaling out applications by distributing the work across many entities, each with a modestly sized state.

Note

Entity functions and related functionality is only available in Durable Functions 2.0 and above.

General concepts

Entities behave a bit like tiny services that communicate via messages. Each entity has a unique identity and an internal state (if it exists). Like services or objects, entities perform operations when prompted to do so. When an operation executes, it might update the internal state of the entity. It might also call external services and wait for a response. Entities communicate with other entities, orchestrations, and clients by using messages that are implicitly sent via reliable queues.

To prevent conflicts, all operations on a single entity are guaranteed to execute serially, that is, one after another.

Entity ID

Entities are accessed via a unique identifier, the entity ID. An entity ID is simply a pair of strings that uniquely identifies an entity instance. It consists of an:

Entity name, which is a name that identifies the type of the entity. An example is "Counter." This name must match the name of the entity function that implements the entity. It isn't sensitive to case.

Entity key, which is a string that uniquely identifies the entity among all other entities of the same name. An example is a GUID.

For example, a Counter entity function might be used for keeping score in an online game. Each instance of the game has a unique entity ID, such as @Counter@Game1 and @Counter@Game2. All operations that target a particular entity require specifying an entity ID as a parameter.

Entity operations

To invoke an operation on an entity, specify the:

Entity ID of the target entity.

Operation name, which is a string that specifies the operation to perform. For example, the Counter entity could support add, get, or reset operations.

Operation input, which is an optional input parameter for the operation. For example, the add operation can take an integer amount as the input.

Scheduled time, which is an optional parameter for specifying the delivery time of the operation. For example, an operation can be reliably scheduled to run several days in the future.

Operations can return a result value or an error result, such as a JavaScript error or a .NET exception. This result or error can be observed by orchestrations that called the operation.

An entity operation can also create, read, update, and delete the state of the entity. The state of the entity is always durably persisted in storage.

Define entities

Currently, the two distinct APIs for defining entities are:

Function-based syntax, where entities are represented as functions and operations are explicitly dispatched by the application. This syntax works well for entities with simple state, few operations, or a dynamic set of operations like in application frameworks. This syntax can be tedious to maintain because it doesn't catch type errors at compile time.

Class-based syntax (.NET only), where entities and operations are represented by classes and methods. This syntax produces more easily readable code and allows operations to be invoked in a type-safe way. The class-based syntax is a thin layer on top of the function-based syntax, so both variants can be used interchangeably in the same application.

The state of this entity is an object of type Counter, which contains a field that stores the current value of the counter. To persist this object in storage, it's serialized and deserialized by the Json.NET library.

Example: JavaScript entity

Durable entities are available in JavaScript starting with version 1.3.0 of the durable-functions npm package. The following code is the Counter entity implemented as a durable function written in JavaScript.

Access entities

Entities can be accessed using one-way or two-way communication. The following terminology distinguishes the two forms of communication:

Calling an entity uses two-way (round-trip) communication. You send an operation message to the entity, and then wait for the response message before you continue. The response message can provide a result value or an error result, such as a JavaScript error or a .NET exception. This result or error is then observed by the caller.

Signaling an entity uses one-way (fire and forget) communication. You send an operation message but don't wait for a response. While the message is guaranteed to be delivered eventually, the sender doesn't know when and can't observe any result value or errors.

Entities can be accessed from within client functions, from within orchestrator functions, or from within entity functions. Not all forms of communication are supported by all contexts:

From within clients, you can signal entities and you can read the entity state.

From within orchestrations, you can signal entities and you can call entities.

From within entities, you can signal entities.

The following examples illustrate these various ways of accessing entities.

Example: Client signals an entity

To access entities from an ordinary Azure Function, which is also known as a client function, use the entity client binding. The following example shows a queue-triggered function signaling an entity using this binding.

For simplicity, the following examples show the loosely typed syntax for accessing entities. In general, we recommend that you access entities through interfaces because it provides more type checking.

The term signal means that the entity API invocation is one-way and asynchronous. It's not possible for a client function to know when the entity has processed the operation. Also, the client function can't observe any result values or exceptions.

Example: Client reads an entity state

Client functions can also query the state of an entity, as shown in the following example:

Entity state queries are sent to the Durable tracking store and return the entity's most recently persisted state. This state is always a "committed" state, that is, it's never a temporary intermediate state assumed in the middle of executing an operation. However, it's possible that this state is stale compared to the entity's in-memory state. Only orchestrations can read an entity's in-memory state, as described in the following section.

Example: Orchestration signals and calls an entity

Orchestrator functions can access entities by using APIs on the orchestration trigger binding. The following example code shows an orchestrator function calling and signaling a Counter entity.

JavaScript does not currently support signaling an entity from an orchestrator. Use callEntity instead.

Only orchestrations are capable of calling entities and getting a response, which could be either a return value or an exception. Client functions that use the client binding can only signal entities.

Note

Calling an entity from an orchestrator function is similar to calling an activity function from an orchestrator function. The main difference is that entity functions are durable objects with an address, which is the entity ID. Entity functions support specifying an operation name. Activity functions, on the other hand, are stateless and don't have the concept of operations.

Example: Entity signals an entity

An entity function can send signals to other entities, or even itself, while it executes an operation.
For example, we can modify the previous Counter entity example so that it sends a "milestone-reached" signal to some monitor entity when the counter reaches the value 100.

Entity coordination (currently .NET only)

There might be times when you need to coordinate operations across multiple entities. For example, in a banking application, you might have entities that represent individual bank accounts. When you transfer funds from one account to another, you must ensure that the source account has sufficient funds. You also must ensure that updates to both the source and destination accounts are done in a transactionally consistent way.

Example: Transfer funds (C#)

The following example code transfers funds between two account entities by using an orchestrator function. Coordinating entity updates requires using the LockAsync method to create a critical section in the orchestration.

Note

For simplicity, this example reuses the Counter entity defined previously. In a real application, it would be better to define a more detailed BankAccount entity.

In .NET, LockAsync returns IDisposable, which ends the critical section when disposed. This IDisposable result can be used together with a using block to get a syntactic representation of the critical section.

In the preceding example, an orchestrator function transferred funds from a source entity to a destination entity. The LockAsync method locked both the source and destination account entities. This locking ensured that no other client could query or modify the state of either account until the orchestration logic exited the critical section at the end of the using statement. This behavior prevents the possibility of overdrafting from the source account.

Note

When an orchestration terminates, either normally or with an error, any critical sections in progress are implicitly ended and all locks are released.

Critical section behavior

The LockAsync method creates a critical section in an orchestration. These critical sections prevent other orchestrations from making overlapping changes to a specified set of entities. Internally, the LockAsync API sends "lock" operations to the entities and returns when it receives a "lock acquired" response message from each of these same entities. Both lock and unlock are built-in operations supported by all entities.

No operations from other clients are allowed on an entity while it's in a locked state. This behavior ensures that only one orchestration instance can lock an entity at a time. If a caller tries to invoke an operation on an entity while it's locked by an orchestration, that operation is placed in a pending operation queue. No pending operations are processed until after the holding orchestration releases its lock.

Note

This behavior is slightly different from synchronization primitives used in most programming languages, such as the lock statement in C#. For example, in C#, the lock statement must be used by all threads to ensure proper synchronization across multiple threads. Entities, however, don't require all callers to explicitly lock an entity. If any caller locks an entity, all other operations on that entity are blocked and queued behind that lock.

Locks on entities are durable, so they persist even if the executing process is recycled. Locks are internally persisted as part of an entity's durable state.

Unlike transactions, critical sections don't automatically roll back changes in the case of errors. Instead, any error handling, such as roll-back or retry, must be explicitly coded, for example by catching errors or exceptions. This design choice is intentional. Automatically rolling back all the effects of an orchestration is difficult or impossible in general, because orchestrations might run activities and make calls to external services that can't be rolled back. Also, attempts to roll back might themselves fail and require further error handling.

Critical section rules

Unlike low-level locking primitives in most programming languages, critical sections are guaranteed not to deadlock. To prevent deadlocks, we enforce the following restrictions:

Any violations of these rules cause a runtime error, such as LockingRulesViolationException in .NET, which includes a message that explains what rule was broken.

Comparison with virtual actors

Many of the durable entities features are inspired by the actor model. If you're already familiar with actors, you might recognize many of the concepts described in this article. Durable entities are particularly similar to virtual actors, or grains, as popularized by the Orleans project. For example:

Durable entities are addressable via an entity ID.

Durable entity operations execute serially, one at a time, to prevent race conditions.

Durable entities are created implicitly when they're called or signaled.

When not executing operations, durable entities are silently unloaded from memory.

There are some important differences that are worth noting:

Durable entities prioritize durability over latency, and so might not be appropriate for applications with strict latency requirements.

Durable entities don't have built-in timeouts for messages. In Orleans, all messages time out after a configurable time. The default is 30 seconds.

Messages sent between entities are delivered reliably and in order. In Orleans, reliable or ordered delivery is supported for content sent through streams, but isn't guaranteed for all messages between grains.

Request-response patterns in entities are limited to orchestrations. From within entities, only one-way messaging (also known as signaling) is permitted, as in the original actor model, and unlike grains in Orleans.

Durable entities don't deadlock. In Orleans, deadlocks can occur and don't resolve until messages time out.

Durable entities can be used in conjunction with durable orchestrations and support distributed locking mechanisms.